Methods and apparatuses are previously known in the art for limiting the sound volume in an apparatus for sound reproduction. One such apparatus comprises a signal source which is connected to a headset, an earphone, a loudspeaker or the like. In such prior art apparatuses, the level limitation is put into effect in that a part of the output signal of the signal source is diverted and led to a detector where the size of the output signal is established. In proportion to the output signal, the detector subsequently generates a control signal which is led to a shunt device for controlling the signal. The shunt device is connected in such a manner that it can shunt a part of the output signal of the signal source past the headset, the earphone or the loudspeaker when the sensed size of the output signal exceeds a given value. This technology operates well as long as the output signal has a voltage exceeding approx. 0.7 V, which is the voltage required for being able to use conventional and commercially available silica based electronics of standard type.
In certain types of signal sources, such as MP3 players, iPods etc., the output signal may at a voltage which is considerably less than 0.7 V. This implies that the prior art technology cannot be applied, since the voltage is too low for causing silica based electronics to function.
Even if the output from a signal source of the above-mentioned type may be as slight as 50-100 mW already at voltages that are clearly less than 0.7 V, this output fed in to an earphone of the earplug type may give rise to extremely high sound volumes. The problem becomes particularly severe if the earplug is passed into the acoustic meatus in such a manner that it seals against extraneous noise. In such situations, sound levels of considerably more than 100 dB may be reached despite the low supplied output.
In signal sources of the type under consideration here, in other words signal sources where the output voltage may be considerably less than approx. 0.7 V, the use is previously known in the art of software-based sound volume limitation arrangements. Such a limitation is integrated into the signal source and requires, for it to function, an earphone, headset or the like which is specifically adapted to the sound volume limitation and the signal source. If the earphone or the headset is replaced, the sound volume limitation arrangement will not function, since the replacement earphone could very well display a considerably higher level of sensitivity than the original earphone matched together with the signal source.
In the relevant type of signal source, i.e. where the voltage of the output signal may be considerably less than approx. 0.7 V, devices are also previously known in the art for limiting the maximum permitted sound volume. Such devices require however a separate voltage source in the form of a battery. Since the sound level limitation must be positioned in the earphone if this is to be able to be used together with an optional signal source, such a solution will be impractical and possibly impossible if the earphone is of the earplug type. Moreover, batteries must be regularly replaced.